Probably ChessMaster 2000 when I wanted a serious game of chess, or
Battle Chess when I just wanted to be entertained. :)
I remember getting ChessMaster back when I was about 12. My parents
couldn't understand why a 'little kid' wanted a chess game, but I
bagged them to get it for me anyway. I not only ended up learning how
to play chess, but became pretty damn good at it too.
CM2K also introduced me to the Franklin Ace and Laser series of Apple
clones because it ended up asking you which type of computer you have
and the Franklin Ace and Laser series were listed there. It wasn't
until much later on that I finally figured out what they were.
I always wondered why it asked though, as far as I can tell they're
functionally the same. Why would the game care?